To reduce the risk of derailment and other potential damage caused by dragging objects, DED systems or “draggers” have been used to detect the presence of objects dragging beneath a moving train. As an example, draggers may be placed at twenty (20) mile intervals over long stretches of a railroad track, in conjunction with other defect detection equipment. If a dragging object is detected, the train is stopped so that the object can be secured to reduce the potential for derailment or other problems. The height of the dragger is determined by balancing the risk of not detecting an object (such as an air hose), which is not dragging very far below the bottom of the train against the likelihood of unnecessarily stopping the train numerous times. For mainline applications, draggers are usually set at a height of about one inch below the top rail so that only objects hanging well below the train will be detected. Air hose detectors, on the other hand, typically extend a couple of inches above the top rail. Consequently, air hose detectors are primarily used in railroad yards rather than open stretches of track so that fast-moving trains will not have to make frequent stops to secure low-risk objects.
One conventional dragger rotates on a shaft between a non-impact position and an impact position. A mechanical contact such as a cam/follower mechanism detects an impact when the dragger is forced into its impact position. The cam/follower mechanism translates the rotational motion of the shaft into a linear motion. The linear motion is used to actuate a conventional switching mechanism to energize an alarm coupled to a switching circuit. For example, a switch, which is closed when the dragger is in its non-impact position, opens when the dragger moves or rotates to its impact position. Moreover, this switch is connected to a relay, which activates an alarm when the switch is opened by an impact, which causes the dragger to rotate.
The conventional switching mechanism employed by draggers described above has several drawbacks. Because it relies upon moving parts, it requires considerable maintenance (e.g., lubrication and adjustment). In colder climates, ice may accumulate in or on such switching mechanisms and inhibit operation of the switch. In addition, moisture and exposure can cause corrosion of one or more of the moving parts, which can result in unreliable operation of the switch.
Other conventional switching mechanisms use proximity sensors to detect the position of an object. However, such switching mechanisms are configured with three-conductor (i.e., wires) for connection to a circuit, and, thus, cannot be used with an existing switching mechanism configured for connection to a two-wire circuit. As a result, to use a conventional switching mechanism having a proximity sensor in connection with a two-wire circuit would require the replacement of the existing two-wire switching mechanism and/or the installation of at least another conductor (i.e., wire) for proper operation.
Thus, there is a need for a switch that relies less on moving parts and that can be used by existing draggers and/or other switching circuit configurations, and that is more reliable.